A hot strip mill normally includes at least a production line and a cooling section that is arranged behind the production line. Alternatively or in addition to the cooling section, a blooming train can be arranged in front of the production line if applicable, or a casting device can be arranged in front of the production line.
The production line comprises a number of roll stands. The number of roll stands can be decided as required. Provision is normally made for a plurality of roll stands, e.g. four to seven roll stands. However, just one single roll stand may also be present in specific cases. A setpoint reduction is specified for each reduction stage that is to be performed at each roll stand, irrespective of the number of roll stands. If a plurality of roll stands are present, setpoint tensions are usually specified for the feed and/or delivery sides. If only one roll stand is present, a setpoint tension may be specified for the feed and/or delivery side. However, this is not necessarily required.
One of the target values that must be maintained in a hot strip mill is the final rolling temperature, i.e. the temperature at which the strip is delivered from the production line. As an alternative to the final rolling temperature, it is also possible to use another variable describing the energy content of the strip at this location, e.g. the enthalpy. The target value should be maintained over the whole length of the strip if possible. The target value can either be constant or vary over the length of the strip.
In order to achieve the target value, the command speed of the production line is normally adjusted accordingly. The command speed is a speed from which the strip speed and the circumferential roll speeds occurring within the production line can be clearly determined, possibly in conjunction with the reductions and setpoint tensions that must be adjusted in the production line. For example, it can be a notional speed of the strip head or the rotational speed of the first roll stand in the production line. The command speed can be defined as a function of the location of the strip head, for example.
Further control elements may be provided in the form of inter-stand cooling devices and/or an induction furnace that is arranged in front of the production line. Like the cooling devices of the cooling section, these control elements act only locally on the strip. The presence of these further control elements is however of lesser significance in the context of the present disclosure. Of critical importance is the command speed (or a variable that is characteristic of the command speed, e.g. the mass flow) and the determination thereof.
As mentioned above, a cooling section is usually arranged behind the production line. In the cooling section, the strip is cooled to a coiler temperature (or coiler enthalpy) in a defined manner. The speed at which the strip passes through the cooling section is defined by the command speed. The adjustment of the cooling profiles that are required for the individual strip points is effected by tracking the strip points and activating control valves, which adjust the coolant volume flow, at the correct time in the cooling devices of the cooling section.
The control valves have considerable delay times in practice, often measuring several seconds. In order to allow the control valves to be activated at the correct time in advance, it is therefore necessary to know at the correct time in advance when a specific strip point will be situated in the region of influence of a specific cooling device. In order to be able to calculate exactly when a specific strip point enters and leaves this region of influence, it is necessary to know not only the momentary value of the command speed, but also the future profile of the command speed, at least in the context of the delay time of the control valves. In addition to this, the throughput time itself, i.e. the time required by the respective strip point to pass through the cooling section, also has an influence on the coiler temperature. The throughput time is obviously also influenced by the profile of the command speed.
The prior art discloses a simplified way of determining the command-speed profile. For example, provision is made for predefining an initial value at which the strip head is to pass through the production line. Provision is further made for predefining an acceleration ramp, over which the strip is accelerated to a final speed as soon as the strip head is delivered from the production line. In practice, this procedure is unsuitable for maintaining a predefined setpoint final rolling temperature (or a corresponding temperature profile) with great accuracy.
The prior art also discloses capturing the (actual) final rolling temperature and correcting the command speed in the sense of minimizing the deviation of the actual final rolling temperature from the predefined setpoint final rolling temperature. This correction can be effected by means of conventional control or (as described in e.g. DE 103 21 791 A1) by means of Model Predictive Control. Irrespective of the type of control (conventional or model predictive), the control intervention (i.e. the modification of the command speed) nonetheless takes place at the same time as the command speed is determined. As in the case of the non-controlled procedure, any prediction is limited to predefining an anticipated future acceleration ramp. It is not certain whether, based on the setpoint and actual values of the next control step, the predicted command speed will actually be accepted. Moreover, the prediction applies to a single control step due to the nature of the system.
Admittedly, this procedure is normally suitable in practice for maintaining a predefined setpoint final rolling temperature (or a corresponding profile) with great accuracy. However, this procedure does not allow the actual variation of the command speed in the next control step to be predicted in terms of direction or value. Any prediction is more of a guess than a true determination.
Moreover, even if the prediction were correct or at least approximately correct, it would be essentially restricted to a single control step according to the teaching of DE 103 21 791 A1. This would be wholly unsatisfactory for timely correction of the control signals for the control elements of the cooling section or of inter-stand cooling devices in the production line. As a result of the variation in the command speed, the coolant volumes that are deposited by the control elements of the cooling section are therefore not deposited on the strip points for which said coolant volumes were previously calculated. This causes deviations in the temperature (or the energy content) of the strip points at the end of the cooling section (e.g. at a coiler) from setpoint set values. The precise maintenance of the final rolling temperature in the prior art is therefore achieved at the cost of significant fluctuation of the coiler temperature, for example.
The prior European patent application 09 171 068.1 (filing date Sep. 23, 2009), unpublished at the filing date of the present application, describes a Model Predictive Control which controls both a production line and a cooling section by means of a prognosis. The mass flow is also predicted in this context. This approach requires coolant volumes that are output by control elements of the cooling section, in order to allow the mass flow to be determined. In addition, the mass flow is also always corrected immediately here. This approach therefore likewise fails to solve the problem of allowing a command-speed profile to be determined reliably in advance.